The overall objective of this research is to advance understanding of the cognitive, linguistic and motoric mechanisms involved in normal and pathologic speech production systems. The specific aim of this proposal is to examine the effects of speaking rate manipulations on phonologic encoding in normal and aphasic persons without dysarthria or apraxia of speech (AOS). Changes in speaking rate may affect phonologic encoding and/or motor level processes. Sound production errors frequently occur in aphasia and are often used as evidence to establish differential diagnosis among types of aphasia and other neurogenic communication disorders (e.g., AOS). The frequent co-occurrence of language and motor level deficits following stroke has caused confusion regarding what types of errors can serve as evidence for disruption of specific levels of the production system. Serial order errors, however, are generally assumed to result from disruption of phonologic encoding processes in some individuals with aphasia who are without concomitant deficits in the speech motor system. Based on a spreading-activation model, it is hypothesized that manipulating speaking rate will affect serial order errors in predictable patterns in individuals with aphasia and in unimpaired speakers. In this repeated measures design, subjects will produce sentences at three speaking rates, in response to auditorily presented tongue-twister stimuli. Phonological-level serial order error (anticipation, perseverative and exchange) ratios will serve as the primary dependent variable.